I am not sure how to access the transformer — most garage door systems have a 110V powerline plug, and I don’t know where to access the internal transformer, or its wires.
That said, it’s interesting to know that the internal transformer is 24VAC — this is the same as sprinkler controllers. Our other product, OpenSprinkler, has internal 24VAC to 5VDC converter, so it would be straightforward to copy the same circuitry to OpenGarage.